A guide to building and understanding the physics of Water Rockets

Version 1.02 June 2007

Warning: Water Rocketeering is a potentially dangerous activity and individuals following the instructions herein do so at their own risk.

Exclusion of liability: NPL Management Limited cannot exclude the risk of accident and, for this reason, hereby exclude, to the maximum extent permissible by law, any and all liability for loss, damage, or harm, howsoever arising.

Contents

WATER ROCKETS

SECTION 1: WHAT IS A WATER ROCKET? 1 SECTION 3: LAUNCHERS 9 SECTION 4: OPTIMISING ROCKET DESIGN 15 SECTION 5: TESTING YOUR ROCKET 24 SECTION 6: PHYSICS OF A WATER ROCKET 29 SECTION 7: COMPUTER SIMULATION 32 SECTION 8: SAFETY 37 SECTION 9: USEFUL INFORMATION 38 SECTION 10: SOME INTERESTING DETAILS 40

Copyright and Reproduction Michael de Podesta hereby asserts his right to be identified as author of this booklet. The copyright of this booklet is owned by NPL. Michael de Podesta and NPL grant permission to reproduce the booklet in part or in whole for any not-for-profit educational activity, but you must acknowledge both the author and the copyright owner.

Acknowledgements I began writing this guide to support people entering the NPL Water Rocket Competition. So the first acknowledgement has to be to Dr. Nick McCormick, who founded the competition many years ago and who is still the driving force behind the activity at NPL. Nicks instinct for physics and fun has brought pleasure to thousands.

The inspiration to actually begin writing this document instead of just saying that someone ought to do it, was provided by Andrew Hanson. Once I began writing, lots of people assisted me, many from the NPL Water Rocket Helpers Team, but I would particularly like to thank, Dave Lowe, Jaco Stander and Gergely Vargha for advice about building launchers, permission to use photographs of their equipment, and for generally putting me right on one or two finer points of rocket design.

Finally, the Water Rocket activity is supported by NPLs management, and I am grateful to both the organisation, and many individuals within them. Their support for this kind of activity is one of the reasons that NPL is such a great place to work.

Thanks to all of you

Michael de Podesta

April 2006

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Section 1: What is a water rocket? At its simplest, a water rocket is basically an upside down fizzy drinks bottle, which has had a nose cone and some fins added. The nose cone The job of the nose cone is to make the rather snub-nosed end of the fizzy drinks bottle more aerodynamic. Also if you have payload on your rocket, or a parachute mechanism, this is probably where it will be placed. The fins Others might disagree, but I think the fins are the parts of a rocket that really give a rocket its character. Technically, the fins are important for ensuring that the rocket flies smoothly

Once we have added the fins and the nose cone, we have something which looks like a rocket. But how do we make it go like a rocket? First we need to add some water, and some kind of release mechanism, that will keep the water in the bottle, until we choose to release it. The water will then leave the bottle through its nozzle. Typically the bottle will be between about one quarter and one third filled with water. Launch To launch the water rocket, we need to pump air into the rocket: this provides the energy for the launch. As the air enters, it bubbles up through the water and pressurises the empty space above the water. You can see that the release mechanism has to be really quite clever, allowing air into the rocket, while not allowing the water to escape until we activate a trigger.

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When the trigger activates the release mechanism, the pressurised air within the rocket pushes the water rapidly out through the nozzle, sending the rocket rapidly into the air.

Peak launch velocities can easily reach 30 metres per seconds (about 60 miles per hour), and without too much difficulty its possible for a rocket to reach heights in excess of 30 m. But launching a rocket straight up in the air can be dangerous

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There are two ways to make your water rocket launch safe. The first way is to use a parachute or other similar device to slow the descent of the rocket. Seeing a rocket launch, reach its peak, deploy a parachute and descend gracefully to Earth, is a really great sight. Unfortunately, its not a very common sight, because getting a parachute to open at just the right time is very tricky, and requires real ingenuity. The second way to safely launch your water rocket is to launch it an angle. Of course, this makes it safe for you, but potentially dangerous for passers by!

One of the best features about launching at angle is that water rockets can travel really impressive distances. Reaching 30 or 40 metres should be quite achievable, but distances beyond 100 m are possible with some careful design. The main problem with launching the rocket at an angle is that the rocket can no longer stand on its own feet, and if it is supported entirely by its nozzle, then it tends to flop over. This happens before launch, and most importantly, it happens just after launch before the rocket has begun to move quickly.

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There are two standard ways to solve this problem: launch ramp and launch tubes. Launch Ramps A launch ramp supports the weight of the rocket before launch and just after launch, until its speed has built up.

Launch Tubes A launch tube is a tube that runs through the nozzle of the rocket. When the trigger activates the release mechanism, the rocket slides along the launch tube before fully attaining free flight. This has two advantages. The first and most obvious advantage is that the launch tube stops the rocket from flopping over just after launch. In this respect it acts like a kind of internal launch ramp. The second advantage is not quite so obvious. Once the trigger has been activated, the high pressure gas inside the rocket expands, and pushes (as the middle section of the figure below shows) the rocket along the launch tube. As it slides along the launch tube it accelerates, and it can be moving quite fast when it leaves the launch tube. However, while it is on the launch tube, it is not losing any water. This gives the rocket a kind of moving start and allows it to use its charge of water more effectively. This can significantly improve its performance.

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Why? So now you know what a water rocket is. But perhaps the question still lingers: Whats the point? The answer is very simple: building and launching rockets is just enormously enjoyable. It combines the simple pleasure of watching in awe at the power of a compressed gas, with the rather more subtle pleasure of mastering an engineering problem. In short, its fun for all ages. The challenge: Some teams, designs and launches from NPLs Water Rocket Challenge

Photo C

redits: Photos from

the NP

L Water R

ocket Challenge W

eb Site. Thanks to Mike Parfitt, Steve Forrester, C

live Scoggins, Stuart R

ogers

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Section 2: How to make a basic water rocket In Section 1, we saw what a water rocket was. In this section well see in detail how to make a basic water rocket that will fly pretty well in a wide range of conditions. Well cover how to launch your rocket in Section 3. 2.1 A Basic Rocket What you will need

A two-litre fizzy drinks bottle: this will form the main body of the rocket. Be sure only to use bottles that contained fizzy drinks: similar looking bottles which contained still drinks (cordial, milk drinks etc.) are not suitable. Fzzy drinks bottles are made from PET (short for Polyethylene Terephthalate), an enormously strong plastic.

A tennis ball, or rubber ball weighing about 60 g. This will form the main part of the nose.

Some corrugated cardboard, or better still, corrugated plastic. This will be used to make the fins.

Duck tape or equivalent strong, sticky tape. Scissors or a knife. Time: Between 30 and 40 minutes

When completed it will look like Schematic Actual

First of all you start with a fizzy drinks bottle. You need to empty out the fizzy drink, get rid of the labels, and rinse it with water. Supermarkets sell value ranges of lemonade and fizzy water that cost only perhaps 20 pence per bottle so this shouldnt cost too much. Now you need to add a nose cone and some fins

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The nose cone The nose cone needs to be slightly pointed, and as well see in Section 4, its also important to have a little bit of weight towards the front of the rocket. My favourite way of achieving both these aims is simply to tape a tennis ball to the end of the bottle.

This might not look quite as aerodynamic as you were hoping for, but trust me, it will fly!

The fins These fins were cut out of an old estate agents For Sale board. More technically, this corrugated plastic (known as Corriflute) is waterproof, and has excellent rigidity for its weight. If you cant find any old For Sale signs, a source of Corriflute is listed in Section 9. However, then there are many suitable alternatives. Corrugated cardboard will do, but does tend to go soggy after a few launches. Also many packaging materials have the same design requirements as water rocket fins (high rigidity-to-weight ratio). One common choice is to cut up old CDs to use as fins. If you do this please then make sure you put tape over any sharp edges in case your rocket should hit someone.

Ive used three fins rather than four, because three fins means one less fin to cut out! Ive included a picture showing the actual dime